GLOBAL ENVIRONMENTAL MODELLING SYSTEMS
Australian Ocean Modelling Software Developers
ABN 28 061 965 339
CHEVRON
Gorgon Development
Measurement and Model Prediction
of
Ocean Currents and Tides
at
Barrow Island,
North Western Australia
October 2005
GEMS – Global Environmental Modelling Systems
Report 375/05
GEMS Contact Details
Melbourne Office
PO Box 149
Warrandyte VIC 3113
Telephone:
+61 (0)3 9712 0016
Fax:
+61 (0)3 9712 0016
Dr Graeme D Hubbert
Managing Director
Mobile: +61 (0)418 36 63 36
Email: [email protected]
Steve Oliver
Director
Mobile: +61 (0)408 81 8702
Email: [email protected]
Perth Office
The Hyatt Centre
3rd Floor, 20 Terrace Road
Perth WA 6000
Telephone:
+61 (0)8 9326 0113
Dr Tony Rouphael
Mobile: +61 (0)400 767 336
Email: [email protected]
Website:
www.gems-aus.com
Disclaimer
This report and the work undertaken for its preparation, is presented for the use of
the client. Global Environmental Modelling Systems (GEMS) warrants that the
study was carried out in accordance with accepted practice and available data,
but that no other warranty is made as to the accuracy of the data or results
contained in the report.
This GEMS report may not contain sufficient or appropriate information to meet
the purpose of other potential users. GEMS, therefore, does not accept any
responsibility for the use of the information in the report by other parties.
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Contents
Introduction ......................................................................................................................... 5
1.1
Scope of Work.................................................................................................. 5
2
ADCP Deployments .................................................................................................. 6
3
Drifting Buoy Deployments ........................................................................................ 7
4
GCOM3D Simulations for the Experimental Period................................................... 9
4.1
Model Description ............................................................................................ 9
4.2
Model Forcing ................................................................................................ 10
4.3
5
6
4.2.1
Meteorology....................................................................................... 10
4.2.2
Bathymetry ........................................................................................ 10
4.2.3
Tides.................................................................................................. 10
Model Predictions........................................................................................... 10
Comparison of Measurements and GCOM3D Predictions...................................... 18
5.1
Tides .............................................................................................................. 18
5.2
Currents ......................................................................................................... 18
5.3
Drifting Buoy Tracks....................................................................................... 18
Conclusions............................................................................................................. 19
Table of Tables
Table 1:
ADCP and Drifting Buoy Deployment Locations ....................................................7
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Table of Figures
Figure 1:
Mooring configuration.............................................................................................6
Figure 2:
Location of the 3 ADCP moorings and the 4 drifter releases. ................................8
Figure 3:
The large grid used to model the propagation of tides and wind-driven
currents and large scale circulations (if any) into the Barrow Island region. .......11
Figure 4:
Near surface currents predicted by GCOM3D at 0200 on 6/9/2005 ....................12
Figure 5:
Near surface currents predicted by GCOM3D at 0400 on 6/9/2005 ....................13
Figure 6:
Near surface currents predicted by GCOM3D at 0600 on 6/9/2005 ....................14
Figure 7:
Near surface currents predicted by GCOM3D at 0800 on 6/9/2005 ....................15
Figure 8:
Near surface currents predicted by GCOM3D at 1000 on 6/9/2005 ....................16
Figure 9:
Near surface currents predicted by GCOM3D at 1200 on 6/9/2005 ....................17
Figure 10: Comparison of predicted (green) and observed (red) sea levels at site
ADCP1. ...............................................................................................................20
Figure 11: Comparison of predicted (green) and observed (red) sea levels at site
ADCP3. ...............................................................................................................20
Figure 12: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP1 (red). ..................................21
Figure 13: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP1 (red). ..................................21
Figure 14: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP2 (red). ..................................22
Figure 15: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP2 (red). ..................................22
Figure 16: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP3 (red). ..................................23
Figure 17: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP3 (red). ..................................23
Figure 18: Progressive vector diagram comparison of near surface currents predicted by
GCOM3D (black) with observations at site ADCP1 (magenta). ..........................24
Figure 19: Progressive vector diagram comparison of near surface currents predicted by
GCOM3D (black) with observations at site ADCP2 (magenta). ..........................25
Figure 20: Progressive vector diagram comparison of near surface currents predicted by
GCOM3D (black) with observations at site ADCP3 (magenta). ..........................26
Figure 21: Wind speed (m/s) for the duration of the ADCP deployments.............................27
Figure 22: Wind direction (deg from) for the duration of the ADCP deployments. ...............27
Figure 23: Comparison of drift tracks predicted by GCOM3D (darker colour) with actual
tracks (lighter colour)...........................................................................................28
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Introduction
Global Environmental Modelling Systems (GEMS) has been contracted to carry
out simulations of the dredging of the Materials Offload Facility (MOF) and the
LNG shipping access channel for the Chevron Gorgon Development at Barrow
Island.
The work is being undertaken using two sophisticated numerical computer
models:
a) The GEMS 3D Coastal Ocean Model (GCOM3D) to simulate the
complex three-dimensional ocean currents surrounding Barrow Island;
and
b) The GEMS 3D Dredge Simulation Model (DREDGE3D) to determine
the fate of particles released into the water column during the dredging
operations.
In order to produce reliable predictions of the fate of turbid plumes during the
dredging it is critical to have accurate predictions of the ocean currents and tides
around Barrow Island. The ocean circulation around Barrow Island is very
complex, mainly due to the shallow reefs and shoals to the north and south of the
island affecting the flood and ebb of the tide from the open ocean on the west to
the eastern side of the island.
Accordingly further field measurements were carried out on the eastern side of the
island to better understand the behaviour of the ocean circulation around the
island. These field measurements involved:
a)
b)
c)
The deployment of 2 Acoustic Doppler Current Profilers (ADCP) by
GEMS for approximately 18 days to cover a full spring to neap tidal
cycle,
The deployment of a third ADCP by MetOcean Engineers; and
The deployment of several drifting buoys which were tracked by boat.
The second phase of the work involved running GCOM3D for the period of
deployment of the ADCPs, driven by tides and Bureau of Meteorology (BOM)
winds, and comparing the model predictions with the data.
1.1
Scope of Work
The Scope of Work was:
a)
b)
c)
d)
e)
Prepare ADCP current moorings
Deploy ADCP moorings off the eastern coast of Barrow Island
Deploy and track a series of Lagrangian drifters
Extract gridded BOM data for the region and time of ocean current
measurements.
Run GCOM3D for the period of ocean current measurements (approx. 3
weeks) driven by winds and tides.
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f)
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Extract data from current meters and analyse to produce time series of
currents at specific depths.
Develop Lagrangian time series from drifter data.
Compare GCOM3D predictions with drifter and current meter data and
produce a technical report.
g)
h)
2 ADCP Deployments
The ADCPs and basic mooring components are shown schematically in the
mooring design in Figure 1.
Buoy
Current
meter
small
weig
ht
40m
float
Large
weight
10m rope
Figure 1:
Mooring configuration
The mooring locations are defined in Table 1 and marked in Figure 2.
The ADCP’s deployed by GEMS were from RDI Instruments in the USA and were
calibrated and supplied by there agent in Australia (Underwater Video Systems).
The work boat “the Gun”, and diving and logistics support for the mooring
deployments was provided by RPSBBBG.
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3 Drifting Buoy Deployments
The Australian Maritime Safety Authority (AMSA) Search and Rescue (AUSSAR)
in Canberra provided wireless tracked drifting buoys (known as SAR datum
buoys) for the lagrangian drifter experiments. These buoys are cylindrical,
approximately 30cm long and 4cm diameter. The heavy battery for the wireless
transmitter sits in the base of the cylinder and as a result only 2-3cm of the
cylinder is above water. Accordingly, although yellow in colour, they are difficult to
see in the open ocean. A thin wireless aerial protudes above the cylinder
approximately 20cm and transmits in the range 119.05 to 119.35 Mhz. A wireless
receiver on deck can then detect the direction of the buoy from the boat and the
signal strength gives an indication of proximity. These buoys have been used
world-wide for many years by Search and Rescue authorities to provide
information on surface currents at incident locations. The SAR buoys are subject
to very low windage due to there design and rely on wireless detection, rather
than vision, for determining there position.
The release points for the SAR datum buoys are defined in Table 1 and the tracks
are shown in Figure 2.
.
Table 1: ADCP and Drifting Buoy Deployment Locations
Instrument Deployment Deployment Duration Average
Latitude
Longitude
wind speed
during
deployment
(m/s)
Average
wind
direction
during
deployment
(deg from)
ADCP 1
-20.72375
115.61598
17 days
n/a
n/a
ADCP 2
-20.83332
115.50960
17 days
n/a
n/a
ADCP 3
-20.89968
115.50013
17 days
n/a
n/a
Drifter 1
-20.83332
115.50960
2 hours
3.8
80
Drifter 2
-20.93920
115.47323
5 hours
3.9
53
Drifter 3
-20.80220
115.48358
5 hours
4.4
60
Drifter 4
-20.80287
115.48422
12 hours
6.0
26
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Figure 2:
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Location of the 3 ADCP moorings and the 4 drifter releases.
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4 GCOM3D Simulations for the Experimental Period
4.1
Model Description
The dominant influences on the circulation in the waters surrounding Barrow
Island are the local wind and tides. This circulation can be simulated to a high
level of accuracy using the GEMS three-dimensional ocean model (GCOM3D).
GCOM3D is a state-of-the-art 3D primitive equation ocean model, which has been
developed by GEMS to study and predict ocean currents on or near the
continental shelf and in harbours and estuaries anywhere on the globe. GCOM3D
includes the non-linear advection terms and is driven by wind stress, atmospheric
pressure gradients, astronomical tides, depth and terrain dependent bottom
friction, and ocean thermal structure (where relevant). For high-resolution studies
over small regions GCOM3D can be nested in larger domains and still runs
relatively fast on any modern computer (PC or UNIX).
For search and rescue applications and the tracking of buoyant discharges the
surface ocean currents from GCOM3D are used. For oil spill modelling, water
quality, sediment transport and other marine discharge studies, which often
require an understanding of the vertical variation of the currents, the full threedimensional current field is used.
GCOM3D is the longest serving three-dimensional ocean model in Australia. It
was the first 3D ocean model to be used on a consulting job (Geelong Ocean
Outfall, 1984) and has since been continuously developed in the research world
and since the formation of GEMS in 1993.
GCOM3D has been used by the Australian Maritime Safety Authority in Canberra,
as the national ocean forecast model for search and rescue, (and oil spill
prediction) for the past three years. During this time the model has been used at
many locations around the Australian coastline and verified against SAR buoys
(surface drifters) with only three cases in three years producing incorrect results.
These cases have since been shown to be due to the influence of the East
Australian Current, which has now been incorporated via the routine inclusion of
satellite altimeter data from the NOAA satellites.
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4.2
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Model Forcing
Model forcing includes both wind and tides concurrently. In addition, as stated in
the previous section, GCOM3D routinely uses satellite data to define forcing from
sea level variations deriving from large-scale ocean circulation properties (such as
the Leeuwin Current on the West Australian coast).
4.2.1 Meteorology
GCOM3D can be driven with gridded atmospheric model output or single station
data. For this study 3-hourly gridded data from the BOM operational forecast
model (LAPS – Limited Area Prediction System) was obtained for the
experimental period.
4.2.2 Bathymetry
The bathymetric data sets held by GEMS were updated with bathymetry acquired
by Chevron. The GEMS database has been developed from a range of sources
including data from Geoscience Australia (formerly AUSLIG) and oil company
surveys. Of particular relevance to this project is that the original 3D bathymetric
survey of the Gorgon field is included together with the Apache Energy 3D
bathymetric survey from south of Barrow Island to the Montebello Islands.
4.2.3 Tides
Tidal forcing was based on data from the GEMS Australian region gridded tidal
data base which has been developed with extensive modelling programmes. The
tidal data for this project was enhanced with data from a high resolution tidal
modelling project carried out by GEMS for Apache Energy in 1998.
4.3
Model Predictions
To verify GCOM3D a bathymetric grid covering the region in Figure 3 was set up
at 500 metre resolution. Tidal data for the model boundaries was extracted from
the GEMS database and winds from the Bureau of Meteorology were used to
force GCOM3D.
A second grid (see Figure 4) was setup on a 100 metre grid surrounding Barrow
Island and nested inside the larger grid (note that for the dredge plume modelling
a higher resolution 50m grid was used)
GCOM3D was run for the month of September 2005, on both the large and nested
grids, producing half-hourly currents at between 5 and 15 levels in the water
column (depending on the depth).
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Figures 4 to 9 show the simulated flood and ebb cycle during spring tides at 2
hourly intervals.
Note that the purpose of these figures is to illustrate the circulation pattern around
Barrow Island and so the current vector scaling in these figures varies for each
figure in order to give a good illustration of the current direction, even at low
current. For a more detailed understanding of the current speeds reference
should be made to the ADCP results and the comparisons with GCOM3D
predictions.
Figure 3:
The large grid used to model the propagation of tides and wind-driven
currents and large scale circulations (if any) into the Barrow Island
region.
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Figure 4:
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Near surface currents predicted by GCOM3D at 0200 on 6/9/2005
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Figure 5:
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Near surface currents predicted by GCOM3D at 0400 on 6/9/2005
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Figure 6:
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Near surface currents predicted by GCOM3D at 0600 on 6/9/2005
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Figure 7:
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Near surface currents predicted by GCOM3D at 0800 on 6/9/2005
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Figure 8:
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Near surface currents predicted by GCOM3D at 1000 on 6/9/2005
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Figure 9:
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Near surface currents predicted by GCOM3D at 1200 on 6/9/2005
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5 Comparison of Measurements and GCOM3D Predictions
5.1
Tides
The MetOcean ADCP did not measure tidal levels so comparisons were only
available at the two GEMS sites. Figures 10 and 11 show the comparison of
measured and predicted sea levels for the 18 days of the deployment at these
sites.
5.2
Currents
Time series comparisons of near surface ocean currents predicted by GCOM3D
with observations at the three ADCP sites are presented in Figures 12 to 17.
An alternative method of comparison is progressive vector diagrams which show
the movement of a particle if the currents at a site were constant throughout a
region. Figures 18 to 20 show progressive vector diagrams for each of the three
ADCP sites comparing model predictions with measurements.
For most of the experimental period the winds were from the north-east, inducing
a wind-driven current residual towards the south. The wind speeds and directions
for the 17 days of ADCP measurements are given in Figures 21 and 22
respectively.
5.3
Drifting Buoy Tracks
Comparisons of drift tracks predicted by GCOM3D with the four drifting buoy
tracks are presented in Figure 23.
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6 Conclusions
The agreement between GCOM3D model predictions of sea levels, currents and
drifting buoy tracks and observations is very good in all cases. Prior to this work
the manner in which the flood and ebb tide circulated on the eastern side of
Barrow Island was not well understood.
In particular, various results have been presented for the meeting of the flood tide
on the east of Barrow Island after flowing around the southern and northern end of
the island. The combined results of the ADCP measurements and drifters now
indicate that GCOM3D is simulating the circulation around Barrow Island with a
good level of accuracy.
Examination of the circulation shows that the shallow bathymetry to the north and
south of Barrow Island restricts the flow of the flood tide from the open ocean to
the coast. Consequently the mass transport of water to the coast, east of Barrow
Island, during the build up of high tide is derived from flows around the north and
south of Barrow Island and from water flowing south-east (varies) in the gap
between the Montebello Islands and the Burrup Peninsular. This latter flow is
induced because of the limited mass transport possible across the shallow areas
to the north and south of Barrow Island.
In the vicinity of Barrow Island the flood tide flows around the north and south
ends of the island and generally meets anywhere between Dugong Reef and the
Lowendal Shelf and then combines to flow towards the coast. The meeting of the
two components of the flood tide is dependent on the spring-neap tidal cycle and
the strength of the southerly winds.
A further outcome of the study is the knowledge that GCOM3D can reliably be
used to simulate currents to study other processes such as the fate of dredge
plumes, or any other marine discharges around Barrow Island.
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Figure 10: Comparison of predicted (green) and observed (red) sea levels at site
ADCP1.
Figure 11: Comparison of predicted (green) and observed (red) sea levels at site
ADCP3.
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Figure 12: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP1 (red).
Figure 13: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP1 (red).
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Figure 14: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP2 (red).
Figure 15: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP2 (red).
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Figure 16: Comparison of near surface East-West current speeds predicted by
GCOM3D (green) with observations at site ADCP3 (red).
Figure 17: Comparison of near surface North-South current speeds predicted by
GCOM3D (green) with observations at site ADCP3 (red).
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Figure 18: Progressive vector diagram comparison of near surface currents
predicted by GCOM3D (black) with observations at site ADCP1
(magenta).
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Figure 19: Progressive vector diagram comparison of near surface currents
predicted by GCOM3D (black) with observations at site ADCP2
(magenta).
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Figure 20: Progressive vector diagram comparison of near surface currents
predicted by GCOM3D (black) with observations at site ADCP3
(magenta).
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Figure 21: Wind speed (m/s) for the duration of the ADCP deployments.
Figure 22: Wind direction (deg from) for the duration of the ADCP deployments.
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Figure 23: Comparison of drift tracks predicted by GCOM3D (darker colour) with
actual tracks (lighter colour).
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